This invention relates to a data processing device, and more particularly, to the protection of a specific program run in a portable data processing device such as an IC card which includes a microcomputer, ROM, RAM, etc.
Banks have recently issued IC cards which have a RAM for storing an account number and transaction data, a ROM for storing a control program, and a microprocessor. The ROM memorizes a password of the card. Only when the password input in a keyboard of a bank terminal unit coincides with that stored in the data processing device, can the card be used, so that illegal use of the card by a third person is prevented. However, illegal use by its owner is not prevented. For example, the ROM stores a coding process program for the communucation between the IC card and the host computer, which must be kept secret from the user (owner of the card). The IC card stores many other secret programs as well. The algorithm of these programs can be discovered if they are run millions of times or more. It has so far been impossible to completely prevent illegal use of the IC card by the owner through the discovery of the algorithms of the programs.